While the invention disclosed herein has application to all forms of internal combustion engines, it is especially adaptable for use on a marine propulsion device such as an outboard motor, and especially a two stroke cycle outboard motor having a throttle and spark advance operated from a twist grip on a tiller. In motors of this type, the twist grip on the end of the tiller is rotated about the axis of the tiller to advance or retard the throttle and spark timing by means of a suitable linkage. Manufacturing tolerances dictate that the linkage be adjustable from engine to engine so that each engine can be set up to achieve optimum performance.
Engines of this type have generally been constructed with various biasing means such as springs which bias the linkage to an idle condition. Such biasing means must be overcome to open the throttle and advance the spark. Moreover, it is generally preferred in two cycle engines that this linkage generally accommodate simultaneous movement of both the throttle and spark timing plate through a portion of the range of motion from idle to full throttle, then hold the spark timing constant while the throttle is further opened. One prior method of accomplishing this motion was by spring loading the spark advance and extending this spring to allow further motion of the throttle while holding the spark timing constant. Previous attempts to reduce the number of springs in linkages of this type have lead to problems in manufacture and adjustability.
It can be appreciated that overcoming these springs, especially the spark advance spring, can quickly fatigue the wrist of an operator. Thus, it is desirable to have an adjustable linkage which can control throttle and spark advance while overcoming a minimum to biasing means.
Attention is directed to the following patents:
______________________________________ U.S. Pat. Nos. 3,721,223 4,566,415 4,071,002 4,606,314 4,528,954 JAPANESE REFERENCE 99,064JA BRITISH REFERENCE 410,420Br. 975,079Br. ______________________________________